The invention relates to a device for monitoring an area of coverage on a work tool.
Work tools of this type generally are systems such as machines, plants or vehicles, and their operating movements can endanger persons within a danger zone. A device as mentioned in the above is used to secure such a danger zone by preventing objects from entering an area of coverage that provides the highest possible coverage for the respective danger zone. In particular, devices of this type are used for securing machines such as bending or folding presses, cutting machines, or punching machines. These machines comprise a first machine part which carries out operating movements in the direction of a second machine part. The first machine part of a bending press is an upper tool which can be moved in a perpendicular direction toward a lower tool forming the second machine part for carrying out a bending operation.
To prevent any danger to persons, the area in front of the upper tool or, in general, the area in front of the first machine part must be monitored, such that the machine is stopped immediately if a person enters the danger zone of the machine.
German reference DE 197 172 99 A1 discloses a protective device for monitoring such a danger zone. This protective device comprises an optical sensor arrangement, formed by a light curtain or a light curtain arrangement, which is moved along with the first machine part. The light curtain, or each light curtain, comprises a transmitter for emitting light rays and an associated receiver, wherein these are installed on opposite edges of the first machine part. The light rays emitted by the (or each) transmitter extend parallel to a lower edge of the first machine part, which extends along a straight line, and impinge on the associated receiver if the protective zone is clear. An object entering the protective zone will interrupt the beam path of at least one light barrier, whereupon the operating movement performed by the first machine part is also interrupted.
To ensure that the protective zone monitored with the light barrier or the light barrier arrangement provides the highest possible coverage of the danger zone, a receiving signal is needed which shows that the transmitting light rays for the (or each) light barrier extend at a specified, safe distance along the lower edge of the first machine part. To adjust this safety distance, an additional light barrier is provided for which the transmitting light rays extend at the level of the lower edge of the first machine part. The optical sensor arrangement on the first machine part is moved for this adjustment operation until a beam interruption is recorded for the light barrier to be adjusted. Following this, the sensor arrangement is moved slightly in the opposite direction until the interruption of the light barrier beam is just canceled.
This protective device has the disadvantage that a danger zone can only be monitored in points along the beam axes of the transmitting light beams for the light barriers. This is a problem, in particular, if small object sizes must also be detected, e.g., a finger on a hand or the like, to ensure adequate protection of persons. A further disadvantage is that the light barriers must be adjusted to be at a safe distance relative to the first machine part for monitoring the chosen danger zone, an operation that is involved and time-consuming.
Finally, it is a disadvantage that these protective devices cannot be used at all or only to a limited degree on machines designed as bending or folding presses. These machines are used to produce three-dimensional work pieces, meaning that following the bending operation, one side of such a work piece can block the beam path of the light barriers, thus no longer allowing the detection of objects.